1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for producing a magnetic resonance image of a test piece by utilizing the phenomenon of magnetic resonance and, more particularly, to an improved apparatus designed to stabilize the static magnetic field applied therein.
2. Description of the Prior Art
In the magnetic resonance imaging apparatus (hereinafter referred to as MRI apparatus), a uniform static magnetic field is applied to a desired region of a test piece, so that magnetic resonance is caused merely in a specific slice portion, where a sectional image thereof is to be obtained, by a transmitting coil to form a radio-frequency magnetic field orthogonally to the static magnetic field, and the MR signal generated from the atomic nuclei after removal of such a radio-frequency magnetic field is detected by means of a receiving coil. In such a structure, a gradient magnetic field having a linear inclination to the X'-axis (coordinate system with rotation of an angle .theta..degree. from the X-axis) is exerted on the static magnetic field to obtain a composite MR signal, and a desired MR image can be formed on the basis of such a composite signal.
In the MRI apparatus mentioned, it is necessary for the static magnetic field to retain satisfactory uniformity in a wide range as well as to maintain high stability. For example, in ordinary proton imaging, the requirement regarding the stability of the static magnetic field is merely 2 ppm/H or so; whereas in chemical shift imaging, the required stability is as high as 0.1 ppm/H. In the case of a superconducting magnet device where a static magnetic field is generated in a superconductive state, a high stability of 0.1 ppm/H is achievable relatively easily under a steady condition. However, at the time of raising the static magnetic field or immediately after changing the intensity thereof, it is extremely difficult to attain the desired stability of 0.1 ppm/H because of intensity attenuation.
Although such attenuation of the static magnetic field intensity can be corrected in a normal conducting magnet device by controlling the current fed to the coil, the correction is impossible in a superconducting magnet device due to adoption of a permanent mode where the device is usually operated with the power source disconnected therefrom.